Travis-CI: install libpcap-dev
Add new application env variables to eunit tests

We now need to set capable_switch_ports and capable_switch_queues before
starting the linc application.
Mock inet:getifaddrs/0 in linc_tests

The application cannot start without at least one network interface with
a non-zero MAC address.
More verbose test failures
Mock inet:getifaddrs/0 in linc_us4_tests
Merge pull request #148 from legoscia/travis-fixes

Travis fix, unit test fixes